1. Field of the Invention
The invention relates to a mercury lamp of the short arc type which has high focussing efficiency and good intensity light stability, and which is usable for a semiconductor exposure device.
2. Description of Related Art
In the exposure process for manufacturing of semiconductors, a mercury lamp of the short arc type which emits UV light with a primary wavelength of 365 nm (hereinafter called "i-line") has recently been used. Since the degree of integration of the integrated circuit of the semiconductor increases each year, there is accordingly a higher and higher demand for image resolution during exposure. Furthermore, due to the enlargement of the exposure surface, as a result of the increase of the wafer aperture or due to the modified illumination technique which is used to achieve high image resolution, there is a need to increase the amount of radiation of the UV light from the light source.
Furthermore, enormous plant investment is necessary to build a production line for semiconductor manufacture. For capital yield for this purpose there is, therefore, also a demand for increasing the throughput in terms of the production amount per unit of time. Therefore, there is a demand for higher radiant efficiency and higher focussing efficiency for the exposure light source.
In a mercury short arc lamp, Xe is ordinarily filled as the buffer gas in order to improve the starting characteristic of the lamp and achieve a heat insulation effect of the gas and the arc tube.
Furthermore, it is has also been proposed that rare gases other than Xe with a pressure of less than 10 atm be filled, for example, as can be taken from the patent disclosure document of Japanese patent application SHO 54-086979. However, these rare gases are filled in many cases as the starting gas to simplify starting of the discharge. The difference of the i-line output characteristic in these types of gases is not sufficiently known.
In a commercial mercury lamp of the short arc type for purposes of semiconductor exposure, besides Xe, Ar or Kr is usually hardly used as the rare gases, or if used at all, filled only at a pressure of less than 1 atm to roughly a few tenths of an atm.
As the result of intensive research by the inventors, it was found that, in a lamp in which Xe is filled as the buffer gas, when Xe is filled at greater than or equal to 8 atm, the spectral width of the i-line emitted from the lamp increases, resulting in a decrease of the image resolution of exposure.
Furthermore, it was found that in a lamp in which Ar or Kr has been filled as the buffer gas, according to the increase of lamp current based on the vigorous motion of the arc, the radiance in the vicinity of the cathode tip fluctuates, that the time fluctuation of the illuminance in the reticle area becomes greater, and that degradation of semiconductor exposure is caused.